Many biologically active compounds require intracellular delivery in order to exert their therapeutic action, either inside the cytoplasm, within the nucleus or other organelles. Selective delivery to particular organs, tissues, cells, or sub-cellular localizations, is highly-desirable to avoid or minimize undesirable side-effects in non-target organs, tissues, cells, or sub-cellular localizations. Thus, the ability to deliver molecules of therapeutic benefit efficiently and selectively is important to drug development.
More than two decades ago it was discovered that certain short sequences, composed mostly of basic, positively-charged amino acids, e.g., Arg, Lys or His, have the ability to transport an attached cargo molecule across the plasma membrane of a cell. These basic sequences are commonly referred to as cell-penetrating peptides (CPPs) or protein transduction domains (PTDs). Prior art CPPs are generally short cationic and/or amphipathic peptide sequences, often between 20 and 50 residues in length, characterized by an ability to translocate across the membrane systems of mammalian cells, localize in one or more intracellular compartments, and mediate intracellular delivery of a cargo molecule e.g., a drug or other therapeutic agent, or a diagnostic agent such as an imaging agent.
Arguably, the most widely-studied and utilized CPP is a peptide derived from the human immunodeficiency virus (HIV-1) transactivator of transcription (TAT) protein. A positively-charged fragment of HIV-1 Tat protein comprising residues 47-57 of the full-length protein penetrates cultured mammalian cells. Since the discovery of Tat, other polycationic CPPs such as e.g., penetratin (a fragment of Antennapedia homeodomain) and vp22 (derived from herpes virus structural protein VP22) have been identified and characterized for their ability to translocate and deliver distinct cargos into the cell cytoplasm and nucleus in vitro and in vivo. Exemplary known CPPs are set forth in Table 1.
TABLE 1Characterized CPPsCell-penetrating peptides (CPP)SequenceOriginAmphipathic peptidesPenetratin (43-58)RQIKIWFQNRRMKWKKDrosophila melanogaster Amphipathic modelKLALKLALKALKAALKLASyntheticpeptide TransportanGWTLNSAGYLLKINLKALAALAKKILChimeric galanin-mastoparan SBPMGLGLHLLVLAAALQGAWSQPKKKRKVChimeric Caiman crocodylus Ig(v)light chain-SV40 large T antigen FBPGALFLGWLGAAGSTMGAWSQPKKKRKVChimeric HIV-1 gp41-SV40 largeT antigen Cationic peptidesHIV Tat peptide (48-60)GRKKRRQRRRPPQViral transcriptional regulator Syn-B1RGGRLSYSRRRFSTSTGRProtegrin 1 Syn-B3RRLSYSRRRFProtegrin 1 homoarginine peptideRRRRRRR (RR)Synthetic(Arg)7 and (Arg)9)
The precise mechanism(s) by which CPPs achieve their cellular internalization has been somewhat controversial. However, there is consensus that most CPPs are internalized via an endocytic mechanism. Several endocytic pathways exist, and clathrin-dependent endocytosis, caveolae/lipid raft-mediated endocytosis or macropinocytosis may be involved. The first step in cellular entry of a polycationic CPP is thought to be an electrostatic interaction between the polycation and negatively-charged heparin sulphate proteoglycan (HSPG) of the plasma membrane. Proceeding on this basis, a charge distribution and amphipathicity of the CPP are believed to be critical factors for cell internalization, possibly affecting an electrostatic interaction between the CPP and proteoglycans on the plasma membrane. Endocytosis of the CPP following contact with the cell surface is believed to be driven by a variety of parameters including the secondary structure of the CPP, the nature of the cargo to which the CPP is linked, (if any), cell type, and membrane composition. As such, cell internalization is a complex and multi-faceted process.
Notwithstanding that certain CPPs may share some common characteristics that facilitate their cell binding and uptake e.g., polycationic and amphipathic sequences, not all CPPs possess sufficient similarity in their primary structure e.g., amino acid sequence, to readily predict their ability to bind to the cell surface and/or enter the cell based on sequence alone. It is not understood how secondary and/or tertiary structure considerations could effect cellular uptake.
Following endocytosis, the internalized CPP needs to escape the endosome to avoid degradation, and to deliver its cargo to an intended intracellular destination. Escape from the endosome may provide a bottleneck to efficient intracellular delivery of macromolecular cargos. For example, the efficiency of endosome escape appears to be low for Tat, penetratin, Rev, VP22 and transferrin e.g., Sugita et al., Br. J. Pharmacol. 153, 1143-1152 (2008). Delivery of CPP-cargo conjugates in liposomes may assist their escape from the endocytic vesicle e.g., El-Sayed et al., The AAPS J. 11, 13-22 (2009). Moreover, the inclusion of fusigenic peptides, such as the HA2 sequence of influenza (Wadia, Stan and Dowdy, Nat Med. 2004 March; 10(3):310-5. Epub 2004 Feb. 8) can also enhance endosomal escape somewhat, although much of the cell penetrating peptides remain in the endosome. There remains a need for CPPs having an ability to escape the endocytic vesicle efficiently following their uptake.
One limitation to the in vivo utility of known CPPs for delivery of drug cargos is their non-selectivity. A generalized uptake of many existing CPPs in vivo may limit their clinical application, particularly where targeted drug action is advantageous or necessary, or where non-specific targeting of an organ or tissue type can lead to unwanted side effects. Notwithstanding that selection of a CPP for the presence of polycationic centres may provide peptides that are able to facilitate initiation of the internalization process, peptides selected for a primary structure that is positively charged may not be cell-selective in view of ubiquity of HSPG and phospholipid in the outer leaflet of cell membranes.
There is presently insufficient diversity of cell-type selective CPPs to provide coverage for many clinical applications involving drug delivery to different cells, tissues, organs and across organ systems. Tight junctions (TJs), basolateral membranes, and apical membranes may function to restrict the passage of CPPs into all cell types, especially when administered intravenously. The blood-brain barrier (BBB) is located at the endothelial tight junctions lining the blood vessels surrounding the brain, and the primary physical and/or pharmacological and/or physiological component(s) of the blood-testis barrier (BTB) and blood-epididymis barrier (BEB) consists of tight junctions between adjacent epithelial cells lining the seminiferous tubules (Sertoli cells) and epididymal duct, respectively. Such physical barriers and/or pharmacological barriers and/or physiological barriers may also be provided by the presence of active transporters and channels at the basolateral and/or apical membranes. HIV-1 Tat-derived peptides, penetratin and VP22 appear to have limited cellular uptake across these barriers and in certain cell types, both in vitro and in vivo. See e.g., Trehin and Merkle, Eur. J. Pharm. Biopharm. 58, 209-223 (2004). Thus, the existing bank of CPPs may not be sufficient to deliver therapeutic cargos to all cell types, suggesting a need for further functional diversity of CPPs.
Safety is a particular concern for the clinical application of any therapeutic agent, and no less so for CPPs that are utilized to deliver a cargo to one or more cells, tissues, organs or across organ systems of the human or animal body. For example, amphipathic peptides may be cytotoxic by virtue of perturbing the cell membrane, e.g., Sugita et al., Brit J Pharmacol 153, 1143-1152 (2008), and it may not be a simple matter to reduce the cytotoxicity of such peptides if their amphipathicity is critical to their interaction with the lipid membrane and subsequent internalization. Similarly, intrastriatal injection of penetratin at 10 μg dosage has been demonstrated to cause neurotoxic cell death, and in vitro delivery at concentrations of 40-100 μM has been demonstrated to induce cell lysis and other cytotoxic effects e.g., Trehin and Merkle, Eur. J. Pharm. Biopharm. 58, 209-223 (2004). Poly-L-arginine peptides have also been reported to induce cell membrane damage, increased permeability of cell barriers and reduce cell-cell contacts between epithelial cells in vitro, to the induce an inflammatory response when injected into the pleural cavity of rat lungs e.g., Trehin and Merkle, Eur. J Pharm. Biopharm. 58, 209-223 (2004). Accordingly, there remains a need for CPPs having low or reduced cytotoxic side-effects relative to known CPPs.
Many of the limitations of known CPPS are a consequence of the processes used for their identification, and their subsequent adoption in the art before adequate testing has taken place to determine their uptake and/or release from the endosome and/or cell-type selectivity and/or tissue-type selectivity and/or organ selectivity and/or ability to cross physical barriers and/or pharmacological barriers and/or physiological barriers, and/or their safety limits.
Phage-display approaches have been successfully applied for the identification of cell-penetrating peptides and are efficient as they can be performed in a high throughput manner with many peptides being interrogated simultaneously e.g., Kamada et al., Biol Pharm Bull 30, 218-223 (2007). Notwithstanding the widespread and successful use of phage display screening techniques for discovery of new CPPs, existing screening methods do not necessarily select peptides for more than the attribute of cellular uptake, and fail to provide validation of cellular internalization or delivery. There remains a need for improved methods for identifying and isolating CPPs.